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Why do we still use pilots to fly airplanes?
With the recent advances in drone and remote control technology, why are there still commercial and military pilots?
I understand that there can be some serious tactical advantages to human pilots in military operations, but for patrol and commercial flights what are the advantages of a human pilot in the aircraft as opposed to a drone or a remotely controlled airplane?

I fully expect aerial photography and powerline patrol to go 100% drone within our lifetime.

@SteveV. I can see that as well!  Anything without passengers very well may in our lifetimes.

A pilot-less aircraft is just another chance to not lose a life.  But sometimes you have to weigh the odds... and I agree that risking one life in an airplane could potentially save many more.

Actually, there are serious tactical advantages to not having the human on board but sitting safely on the base in military operations. That's why they use the drones (a drone is not autonomous, it needs human operator at the remote controls all the time).

Simple answer: because we trust humans than machines.

I would think that a drone would never be capable of things some amazing pilots can do in the face of disaster. See http://en.wikipedia.org/wiki/Gimli_Glider

There is at least one psychological factor at play here: Most folks today simply don't trust automation at that level, and a good number of people want the company ferrying them around to have "skin in the game" (an employee with the skills to handle an emergency who will do whatever is necessary to save themselves, and thus the passengers too). We may get past that at some point though: We already accept computer-controlled trains, often with the false-logic of "trains run on tracks, so not much can go wrong")

I think any aircraft that carries people on it will always have a pilot. A drone is better because it is smaller, lighter, it can pull 50gs, and it doesn't need life support systems. This is bad news for fighter pilots, and some cargo pilots. However, as long there are airplanes big enough to carry people that require the space, the life support systems, and the like you might as well have a pilot on board you aren't really saving much spending 10x on automation and selling one or two more tickets. And then of course there is this:

One of the main reasons the military uses drones is because they can then have zero people on board, which gives a whole lot of advantages over having any people on board (chiefly: if there's an issue and the drone crashes, it's just a monetary loss, and no expensive personnel die, so you don't have to worry as much about preventing crashes). OTOH, with an airliner full of passengers, cutting a pilot doesn't let you remove the risk of death in a crash, nor save much weight; an automatic airliner couldn't be much different than a normal one, while drones look VERY different from planes.

@voretaq7 there is less to go wrong when the automation only has to think in 1 dimension (forward-backward) than in 2 or 3 dimensions

@asawyer Gimli Glider was exactly the kind of pilot error a computer would not make in the first place. Worst argument ever : )

@Agent_L: I take it you're not in programming? It's turtles all the way down - here's a very similar error made by an (improperly programmed) computer: https://en.m.wikipedia.org/wiki/Mars_Climate_Orbiter

@Piskvor Mars Orbiter is quite unique piece of software that can never be tested until it actually crashes the craft. I actually am in programming, as stated in my profile.

No true Scotsman, eh? But the point I was trying to make stands: only a properly programmed computer with proper inputs would avoid such errors. Otherwise, GIGO. Case in point: Air France 447. (Sorry about the personal jab - your previous comment sounded like "computers are less error-prone than people due to Magic"; I now see that I have attributed that meaning in error)

@cpast with an airliner full of passengers, cutting a pilot doesn't let you remove the risk of death in a crash - then just replace the passengers with robots, duh!!  ; )

Pilots can't be hacked remotely.

Corollary: now that we have 3-D virtual reality, why do still have flight?

That's a good question, and brings up a joke that many pilots know well:

"What's going to be in the cockpit of the future?"
"A dog and a pilot."
"A dog?  Why a dog?"
"Well, the dog is there to keep the pilot from touching anything."
"Ummm, why have the pilot then?"
"Well, someone has to feed the dog! "

Technology has improved to the point where airplanes can pretty much fly and even land themselves.  They are even getting pretty good at handling "normal" emergencies like engine failures and depressurizations.
Where pilots really shine though are the type of abnormal and emergency situations that aren't "in the book".
When an engine flies apart and shreds the hydraulic lines on all three redundant hydraulic systems, a computer will be out of options because it was never programmed to handle that (because it was considered "virtually impossible" when it was designed).  A pilot on the other hand can analyze and experiment until he finds enough of a solution to keep the airplane in the air as he works through other issues.
During a total electrical failure, the computers won't be much good.  Sure, you can design redundant systems and make it so that it "can never happen", but what about when it does?
There are also all of the little things that a pilot is constantly making decisions on how to handle.  Some examples could include:

Can the airplane fly with a particular feature inoperative?
What about multiple features inoperative?
How do they interact with each other?
What do you do when the passengers take too long to board and you are going to miss your departure slot?
What do you do when the ground crew forgets to close a door?
What about when the tug driver doesn't push you back quite far enough?
Or ATC wants to change you to a different runway?
Or you ask ATC to change you to a different runway because it might save a few minutes and that will keep you from having to deice again?
Or another airplane unexpectedly pulls out in front of you while taxiing?
A pilot can see "ugly" clouds off the end of the runway and decide that it would be better to wait a bit before taking off.
What happens when a flock of birds suddenly appears during the takeoff roll?
What happens when a passenger gets sick or unruly?
When do you need to divert and when can you continue?

On a typical flight, a pilot will make hundreds of small decisions that can make a big difference in the flight.  Any one of these (or one of the tens or hundreds of thousands of other things that could happen) could trip up a computer.
The pilot is also there as the "final authority".  If a hacker were to somehow hack into the automated systems, a pilot can always disconnect them and fly the airplane manually.  He can turn off systems, he can be...  Well, the pilot can be creative and a computer can only do what it was programmed to do.
Right now when a drone crashes it isn't such a big deal because nobody was on board.  If an airliner with a load full of paying passengers crashes it is a big deal.  Even just one.
Many of these problems can be overcome.  Some of them already have been.  Some of them may never be fully handled in an automatic way but maybe we could still do automated flights with an acceptable level of safety.  Some automation is actually better than some pilots in certain areas.
However, the biggest reason that we don't have fully automated passenger airplanes is because the general public feels comforted by a person being up front who can take over and carry them to safety if needed.  Even if the technology were 100% ready for automated passenger flights, I think that the number of people willing to purchase a ticket would be so low that it would be financially unfeasible for an airline to deploy it.

Wow, thanks for such a thoughtful answer.

@KeeganMcCarthy You're welcome.  It turned out to be a little longer than I thought it would be, but hopefully my point is clear!  :)

This calls to mind a similar question from scifi.SE: http://scifi.stackexchange.com/questions/35447/futurama-why-does-the-planet-express-ship-need-a-pilot/35455#35455

"Even if the technology were 100% ready for automated passenger flights, I think that the number of people willing to purchase a ticket would be so low that it would be financially unfeasible for an airline to deploy it." Until we get use to the idea, of course. :)

@landroni: If Futurama is any indication, that won't happen for at least another thousand years. ;)

@landroni Absolutely, but I think that may still be a long way off.  I won't be surprised to see a single pilot airplane as automation continues to improve, but I doubt that we will see airlines operating without pilots in my lifetime.  :)

@landroni I think passengers would be more likely to accept it than pilot's unions and government regulators... And the former especially have a lot of power (including with the latter).

Also good to mention that drones are still piloted, just remotely. You can have just a few pilots working a number of drones since they all aren't used simultaneously. On the flip side, commercial flight gives quite a bit of work to the pilots, that you're not really saving on manpower.

@jwenting:  I disagree.  When the first loss-of-life accident in a commercial airliner occurs (and there will be one, eventually), the concept is dead.  Competing airlines will advertise the safety of having a pilot on board and - rightly or wrongly - the general public will shun the pilotless aircraft. "Do you want to fly with a machine in control of your life, or with Sully at the controls?" UNLESS there is a meaningful difference in the ticket price.

Something to keep in mind is that programming a plane to handle every single possible incident that could ever possibly come up may be more labor/time intensive than just training a human and allowing it to assess and creatively deal with new problems when they arise.  Programming and program maintenance aren't exactly simple, and if it takes a team of a few thousand people to write the software...why not just train some pilots?

@JayCarr: Well, in that case it's kind of obvious.  You only pay to write the program once (okay, and then you have to maintain it) compared to hiring 10's of thousands of pilots and paying them every single day.  The software is already being written anyway.  :)

@Lnafziger - lol, mebbe... I write software for a living. Though, admittedly, not in the aeronautical sector.  I can't help but think writing and maintaining (especially maintaining) that code is going to be a lot more time intensive than is generally realized.  There will come a point when covering every instance is going to be more money than it's worth...  Though, again, this is just from my limited perspective.

@JayCarr Oh no, I 100% get that, but perhaps you are misunderstanding the scope of the labor savings here.  There are roughly 100,000 airline pilots just in the US right now.  The math on that adds up very quickly, especially when you consider the year over year over year... savings that we are talking about.

@Lnafziger - hmm, this little sub question gets slightly more interesting as we go along here.  Microsoft, at the moment, has 100,000 employees.  Not all are software engineers, I know, but still...  I wonder how big a company that produced full automation software for aircraft would get.  I honestly am not sure, the automation we do right now is so fundamental, I don't know how intense it will get if we try to automate every last thing.

@JayCarr Microsoft also has a lot of different projects that they are working on and supporting.  Also, it doesn't all have to pay back overnight.  That airplane will be flying without pilots for 30+ years.  In any case, this is getting long and off topic for comments.  Stop by [chat] sometime (I won't be there tonight I'm flying) and we can talk some more!

I, for one, would feel much more comfortable having my life in the "hands" of a computer. A human is way more likely to make an error, although it's also more likely to be able to correct it.

@1ace Each one monitoring the other with the ability to override when needed seems to be a good way to get the best of both worlds.

@SkipMiller remember when Airbus came out with "plastic airliners" and airlines (and competitors) advertised with their aircraft being made of metal and therefore being superior? There was a scant doing the rounds (no doubt Boeing didn't mind) "if it ain't Boeing I ain't going". The initial crashes of Airbus aircraft did nothing to cause negative sentiment. And the higher degree of computer control of their aircraft also didn't cause people to shun them, even after those computers caused multiple high profile crashes (including at le Bourget in front of a crowd of journalists).

Beautiful post, +1! IMO some of the greatest landings in face of certain death are UA 232, BF 9, QF 32, and TACA 110.

I have absolutely no aviation experience, but I was surprised by the question of "What happens when a passenger gets sick or unruly?" I always thought that was up to the cabin crew, not the flight crew?

@raptortech97 The cabin crew is the "first line of defense", but the captain is responsible for everything that happens on the aircraft, and more importantly, if it is something that can not be handled appropriately and the aircraft needs to divert to the nearest suitable airport, then obviously the flight crew will need to be involved.

The ill/unruly passenger problem could be solved. For example, you could give the head flight attendant additional training and authority and a "divert to nearest airport" button they can press. That's basically what they do now, except it's an intercom button instead.

@MattNordhoff As I say in my answer "Many of these problems can be overcome...."  :-)

But if you're getting rid of pilots, why not replace the entire flight crew. If computers are flying the plane, why do you need flight attendants, too? :-)

" Even if the technology were 100% ready for automated passenger flights, I think that the number of people willing to purchase a ticket would be so low that it would be financially unfeasible for an airline to deploy it." - automated metro trains are already in use in many cities.

@Anixx They are indeed,but when they break down they generally coast to a stop.... Rather unlike an airplane.

I know this is old, but your profile shows youre still active. The question mentioned remotely manned vehicles as well. Did I miss somewhere in your answer that explained why we dont just set up a pilot in a virtual or semi-virtual cockepit at base and put a camera in his seat, put some VR goggles on him, make the camera in the real cockpit turn with his head, and replace the windows (green screen replacing glass in his fake cockpit) to show what he'd see in the cockpit in real-time, let him control and stream the controls to the aircraft?

@Viziionary Well, it still comes down to some of the same problems in that what happens when the datalink fails?  I remember reading about a push by someone for the FAA to approve this exact scenario for the second pilot (a "virtual" copilot who could hop from flight to flight as they finished), but they would leave one pilot in the airplane just in case something went wrong.

@jwenting Easyjet use airbus exclusively and haven't had a fatal incident ever.. in over 30 years of business. Can't say the same for companies using Boeing.

@Cloud Easyjet was an all-Boeing operator until a few years ago. By the time I wrote that Easyjet in fact was an all-Boeing operator or was just receiving their first Airbusses.

To play devil's advocate, there were two illustrative examples in 2009:

US Airways Flight 1549 was safely landed on the Hudson, a feat that the autopilot could not have handled; but
Air France Flight 447's autopilot handed control back to the pilots, and their incorrect reactions contributed to the loss of everyone on board.

The idea of the pilot "up front who can take over and carry them to safety" is comforting, but not necessarily accurate. It's worth noting that Chesley "Sully" Sullenberger, the pilot from Flight 1549, was a former USAF pilot, arguably better trained to deal with the situation than the average commercial pilot.
Although it is accurate to say that a pilot can respond to situations an autopilot isn't programmed for, it is not accurate to say that an autopilot couldn't be programmed to deal with extreme situations. I can't lay my hands on it now, but I remember seeing a video where an autonomous pilot system corrected for the explosive removal of a model plane's wing, testing the control inputs and corresponding outputs to adapt to the loss (NASA's Adaptive Control seems similar).
I suspect we will ease into full automation of aviation, via the equivalent of today's drone pilots; imagine a group of pilots, ready to remotely take over in circumstances beyond those autopilots can handle, at some ground location, able to patch in to the airliner remotely. Freight will likely be an earlier adopter on cost grounds, with passenger following once a sufficient safety level is demonstrated.
Equally, automation outside the cockpit has further to go; some of Lnafziger's scenarios would not occur if the ATC was also autonomous, for example. As in other forms of transportation, there may be issues with autonomous and conventional vehicles operating alongside one another that would have to be ironed out.
I agree that the social aspects are far more pressing here than the technological ones, though; people's acceptance of autonomous technologies seems set to lag their performance.

An example of fault tolerant flight control: http://tinyurl.com/op7s2f7 & http://tinyurl.com/p468bwm.  Fly by Wire control systems were designed in such a way that they were able to reconfigure themselves to deal with a damaged aircraft. While the pilot was still in control, The same could of course be done for fully automated aircraft.

Oh, I agree and although I didn't focus on it (because the question was asking why we have pilots), I did say "Some automation is actually better than some pilots in certain areas."

While a computer could be programmed to handle almost anything, there are still the things that happen which were deemed "impossible" or that nobody even thought of in the first place, or the bug in the software that causes a problem.  Remote pilots are a great idea until something goes wrong with the datalink.  You will never convince some people that it will be bulletproof.  +1 though, good points for the discussion!

So what you're saying is we need more USAF pilots so they can proceed to fly commercial when they're done with the Air Force. I can think of 535 people who probably agree with you.

The real problem with AF447 seems to have been that the computers deemed all sensor inputs invalid. (See 02:12 UTC.) This is the "impossible" scenario. If the stall warning hadn't sounded when the pitch was actually decreasing, or otherwise provided information to the flight crew to indicate that they were acting to correct rather than worsen the situation, the outcome may very well have been quite different. A human could have made such a judgment; the computer obviously was not programmed to do so, and so never could have.

@MichaelKjörling true, but my point is that having human pilots there as backup didn't really work out as intended.

Probably (to my mind) because the computers provided insufficient information to the pilots. I'm not familiar with the intricacies of that type of aircraft, but suppose the computer, when sounding the stall warning, would also prominently display the AoA on one of the main cockpit screens. At that point the human could say "oh look, AoA is positive and going down despite the stall warning, and our rate of descent gives ample time to get it managable", keep pitching down, and deal with the altitude loss later once the aircraft was in a safe attitude.

A computer would have to be programmed for that specific scenario in order to handle it; given relevant information (which can also come from even something as simple as looking out the cockpit windows; "where is the horizon?!"), a human can improvise.

@MichaelKjörling Computers also can improvise.  Adaptive algorithms are currently in use all around the world to handle problems like spam filtering, playing Jeopardy!, virus detection, autonomous vehicle driving, freight optimization, etc.  Furthermore, "impossible" failures can happen with humans, too.  Heart attack, stroke, seizure, etc. could strike simultaneously and incapacitate both pilot and copilot, and then our double redundancy would not be enough.

@Nick2253 "... could strike simultaneously and incapacitate both pilot and copilot" Sure, but how often does that happen, compared to, say, software problems of various kinds? Now multiply that software's complexity by some large factor; what are the odds that it will operate error-free in potentially extreme situations? You can always come up with scenarios where any given preventative measure will not be enough to prevent a given failure, but at some point we have to decide what is "good enough". I work as a system developer, and I definitely don't believe what I do is free of errors.

In most of the examples you list, failure of the system is an inconvenience; it does not directly risk the lives of several hundred people both in the aircraft and possibly on the ground. An email being wrongly determined to be spam (or not spam) is hardly a life-ending event. An autonomous ground vehicle could be turned off and it won't, at least, fall out of the sky. And so on.

@MichaelKjörling As a systems developer, I'm assuming what you do is full of errors because "good enough" in your case allows you to have errors.  But the adaptive software algorithms that drive the Curiosity rover on Mars, or pilot autonomous rocket ships do not allow errors.  You've also created a terrible straw man here:  you explain that existing "good enough" standards allow for errors, and since they do, we must assume that more stringent standards would be no different.  But currently, we don't depend on software to fly a plane, so we don't build software to that level of robustness.

You do have to deduct all the accidents caused by pilot error from the ones where a skilled pilot saved a crippled aircraft, though?

In all fairness, AF447 was the result of bad cockpit design (independent side sticks) and poorly trained pilots. If you watch the PBS special on this flight, which came out before the wreck was found but the conclusions turned out to be remarkably accurate, a competent pilot can handle loss of airspeed indication at cruising altitude.

If we compare the route to improvement for AF447 and US Airways 1549: there's on the one hand the quite manageable suggested improvements post AF447 (better pilot training, revised procedures, minor redesign of equipment) versus implementing an autopilot capable of safely ditching the airplane in a waterway without any engines available on the other hand. Call me pessimistic, but I don't think that we will see the latter one materialize. First, it's a tall order and secondly, how do you test that? If you sell a Sully-Bot 1.0; you're bound to test to get certified - and that's a huge cost.

In addition to Lnafziger's excellent post which focuses on the advantages of a human in the cockpit, I will focus on the relation between recent developments and the fact that there are still pilots in the cockpit. In commercial aviation, technological changes take a very long time before they are widely adopted. Especially if they require complex changes to the whole industry.
Suppose some manufacturer would start to develop a unpiloted transport aircraft today. Just the design of the aircraft and the systems would probably take about 10 years. In parallel certification standards need to be developed. It will probably take another 5 years to certify the aircraft. That would mean that in an optimistic scenario it would take 15 years before the first commercial flight could be made with such an aircraft. During this time conventional piloted aircraft will still be produced. And they will still be produced for a while after unpiloted aircraft becomes available. Now consider that an airframe will be flown for about 30 years, perhaps a little less if it really uneconomical to do so. That means that we will see piloted aircraft for at least another 50 years if we'd start to move to unpiloted commercial aviation today.
This also means that for a long time there will be a mix of piloted aircraft and unpiloted aircraft together. How will ATC deal with that? For ATC to deal with unpiloted aircraft, a lot of changes need to be made to ATC systems worldwide. These will take even longer than the development of the aircraft itself.

Very true.  Look how long it is taking just to allow UAV's into the US NAS!

The control loop for an aircraft in IFR is fairly baroque:

the control system receives the position of the aircraft (traditionally by radar, nowadays by transponder)
an air traffic controller observes the position and decides on a velocity change to maintain safe and efficient airspace usage (possibly with computer assistance?)
the ATC gives the pilot a voice command
the pilot confirms and adjusts the autopilot

Why not just have the ATC computer message the plane's computer?

Current manufacturer initiatives
As to the current state of fully autonomous passenger jets technology (in 2017), there is some interest by manufacturers in the technology, particularly by Boeing, but it would be a stretch to say that manufacturers are working hard to get fully autonomous passenger jets commercially ready soon.
At the Paris Airshow in 2017, Mark Sinnett, vice president at Boeing responsible for innovative future technologies, clarified that, as paraphrased by the Wasington Post, "The artificial intelligence that Boeing will test will be capable of making decisions normally made by pilots. Sinnett said that the technology could be used to reduce the number of pilots required for long-haul flights or, in some situations, allow fully autonomous flying"
Earlier in 2017 Sinnett had stated that  “We are not smart enough to preprogram all those things. The machine has to be capable of making the same set of decisions [as a pilot],” Sinnett said. “If it can’t, we cannot go there.”
In a talk at the American Institute of Aeronautics and Astronautics SciTech Conference in January 2015, John Tracy, Boeing’s chief technology officer, said: “Some of our freighter customers are asking us for those [autonomous airplane] systems today.”
He went on: “We are quite confident that technologically, the toolkit is filled. With respect to a commercial airplane, there is no doubt in our minds that we can solve the problem of autonomous flight. It’s a question of certification
procedures, regulatory requirements and, even more significantly, public perception. "
A spokesperson for Airbus said in 2016: “Airbus is not developing an autonomous airplane.”
And a spokesperson for the FAA, which would be responsible for certifying an autonomous plane, said in 2016 that: “The FAA has no current unmanned aircraft certification projects in the transport category, nor has anyone engaged the agency on such a project.”
Chinese company Ehang has produced a passenger drone, the Ehang 184, that can fly a 25 minute preprogrammed route from take-off to landing. This doesn't preclude many of the issues described below. The technology is still in test flights in 2017.
Public Perception
While previous advances in autopilot and even crashes due to misuse of autopilot have not caused widespread distrust, John Tracy of Boeing stated that public perception is very important to successful implementation. My opinion is that when things start to go wrong in the cockpit, many people would trust a human with thousands of hours of experience over a computer with millions of hours of testing. The lack of faith in self-driving car technology seems to confirm this tendency.
What decisions currently require a pilot?

Preflighting the airplane: The pilot needs to review the entire plane before flying. This includes reviewing the list of items that need non-urgent repairs and deciding if he's comfortable with the safety margin on the plane. Sometimes this relies on predicting whether the plane will encounter certain weather conditions.

Weather:  A pilot has to make the decisions on whether or not a route is safe to fly or if conditions will get worse. In-flight, this may require a decision to declare an emergency.  A pilot with detailed radar-based weather maps may even be able to choose a route right through a gap between dangerous storm cells.  A computer would have to be better  than a pilot at predicting how safe a particular route is, in order to avoid situations like Southwest Airlines Flight 1248 or many other crashes where weather was a factor.

Flying visual approaches:  Even though autoland technology has existed since 1964, it's still uncommon to have an airport equipped with all runways enabled for zero-visibility approaches. (see this question).  Less than 1% of all landings are currently done using autoland.

Taxiing:  Taxiing is currently done manually, and by visual reference.  Detailed GPS maps and indications of where you are on the taxiways exist but the plane can't drive through them automatically.  Automatic taxiing by use of external towing vehicles is an area of active research (see here or here)

Communication with ATC:  Airspace clearances, weather information, taxiway directions, traffic diversion, declaring an emergency and more are all handled verbally with ATC.  Partial replacements for much of this has been made with ACARS, TCAS, and advanced radios, but there remains a lot of work to be done in taking ATC verbal communication completely out of some planes.

Avoiding VFR traffic and terrain:   Not all traffic is transponder-equipped and not all terrain is recorded in an accurate database or covered by an enhanced ground-proximity awareness device, so we rely on pilots to avoid these. Inaccurate charts are rare in modern times so the risk here is probably acceptable, but imperfections in maps have contributed partially to crashes like Lockheed L-1329 JetStar N400M or World Airways Flight 802

Identifying when things are going wrong: Although some malfunctions, like total engine failure, are easy to identify, other issues are trickier to sort out and require visual confirmation or pilot judgement.  For example, if two flight instruments disagree, which one is malfunctioning?

Making a plan of action for equipment malfunctions:  Can a plane still fly to an airport with an equipment problem or do you need to declare an emergency?  Is an unusual flying technique or reduced flight envelope required after the failure? Not to be an alarmist, but exploding engines, landing gear failures, equipment fires, and tire explosions can happen several times in the industry in a single year, not to mention much more common issues like indicator lights not working, bird strikes, compressor stalls, engine malfunctions, and autopilot disengagement.

Route planning:  Pilots still often manually read approach charts and sectional charts and enter the resulting route into the FMS.

Running some of the manual troubleshooting: Current hardware doesn't allow the flight control computers to pull breakers, disengage generators, or restart engines.   Currently the philospophy that I've seen in autopilots is "if something unusual is going on, disengage and pilots will take care of it".

Turbulence and windshear:  Autopilots can currently handle these tough wind conditions, but pilots hand-flying the aircraft are usually better and provide a larger margin of safety.

On-board medical problems and other passenger problems:  Someone needs to make a judgement call as to whether an onboard medical problem or someone getting violent over yoga would require a diversion, and currently this decision is up to the pilot and other flight crew.

Can't we handle some of this remotely?
You may ask why we can't simply remotely handle some decisions from mundane weather rerouting to emergency procedures, but that only eliminate some of the hurdles. Currently the technology for remote piloting is not mature enough (see these questions about engineers on board, converting planes to drones, and uploading FDR data).  I've seen some of the latest generation  connectivity technology for aircraft and even in 2017 it still has reliability, latency, cost, or speed issues that make it unsuitable for handling a matter of life or death, particularly in area like oceans where satellites are your only option.
The topic of advances towards fully autonomous aircraft is a broad and complex topic.  Here are some articles for further reading:

How Boeing plans to test the technologies for future self-flying
passenger airplanes

This is the Biggest Factor Keeping Planes from Flying Themselves

Pilots Could Soon Lose Their Jobs to Robots

When Will We Have Unmanned Commercial Airliners?

Would you fly in a pilotless airliner

Excellent answer!!  I'm sorry that I didn't see it sooner.

Surely the main reason is moral hazard
The pilot is probably literally going to be the first to die seeing as he's sitting at the front so he's reasonably well motivated to do the job properly
I remember a friend of mine who was an army aircraft technician was obliged to go on all test flights after he'd completed a repair for the very same reason.
I think the concept of civilian drones will die (and probably some poor people on the ground along with it) following the first crash of such a vehicle.

I don't think this is the main reason, although it is certainly a consideration. I think the last paragraph is, however, completely wrong

Even if the automated pilot crashes and kills all aboard, it will have saved two lives.  :-/

Even for military applications, having fully automated aircraft isn't the best of options (except when it comes to cost, pilots cost more than computers).
Quite apart from the risk of accidentally losing control over the drone (a solar flare at just the wrong moment, oops), there's the risk of the control link being jammed or even the control systems being hacked and taken over by an opponent.
While that's not that great a risk sending a Predator drone over Afghanistan of Somalia, sending the equivalent of a B-2 piloted by computers on a strike to say China does invoke that risk.
And I'm not talking about Terminator style robots taking over the world and hunting humans to extinction (which is a big reason why the public has trouble with armed drones) but human interference causing the drone to either crash, divert to an enemy base and be captured, or be reprogrammed to strike back at its owner by the enemy.

And in part that plays in commercial aircraft as well. A well equipped criminal gang could break into the control systems and take them over, sending an airliner with several hundred passengers out of control (or into a holding pattern, demanding ransom). No more trying to sneak people with funny accents onto airliners with plastic explosives in their shoes or underwear, which (romantic though it may be to some, and providing martyrdom to the 'volunteer') isn't very likely to succeed.
And that was already a real worry when Boeing and Airbus introduced their Electronic Flight Bag equipment, the idea that someone could break in in an airliner's systems and reprogram those to provide false data to the pilots who'd then use that to fly to the wrong place caused serious headaches (and those things can be reprogrammed only on the ground using a cable from a laptop...).

This has nothing to do with having a pilot or not. If you have access to the flight systems, you can already crash a fly-by-wire plane. The security threat is well known and dealt with already (at least Airbus has a dedicated team working on ensuring the right code is running on its planes, I bet all manufacturers do the same). At least with automated aircrafts, the pilots can't crash their own planes.


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What type of hoops do I have to jump through to launch high power rockets?
If I want to launch high power rockets NOT at a NAR or TRA sanctioned launch, assuming I am properly certified, what do I need to do?  I looked in the FAR|AIM, but I couldn't find anything on rockets, but I think the rocketry section has been omitted from the PPT FAR|AIM.

The FAR section that you are looking for is available on the ecfr website:
PART 101—MOORED BALLOONS, KITES, AMATEUR ROCKETS AND UNMANNED FREE BALLOONS
More specifically, it is Subpart C— Amateur Rockets
The main regulations that apply are (see the above link for the full text of the regulations):

§101.23   General operating limitations.
(a) You must operate an amateur rocket in such a manner that it:
(1) Is launched on a suborbital trajectory;
(2) When launched, must not cross into the territory of a foreign
  country unless an agreement is in place between the United States and
  the country of concern;
(3) Is unmanned; and
(4) Does not create a hazard to persons, property, or other aircraft.
(b) The FAA may specify additional operating limitations necessary to
  ensure that air traffic is not adversely affected, and public safety
  is not jeopardized.
§101.25   Operating limitations for Class 2-High Power Rockets and Class 3-Advanced High Power Rockets.
When operating Class 2-High Power Rockets or Class 3-Advanced High
  Power Rockets, you must comply with the General Operating Limitations
  of §101.23. In addition, you must not operate Class 2-High Power
  Rockets or Class 3-Advanced High Power Rockets—
(a) At any altitude where clouds or obscuring phenomena of more than
  five-tenths coverage prevails;
(b) At any altitude where the horizontal visibility is less than five
  miles;
(c) Into any cloud;
(d) Between sunset and sunrise without prior authorization from the
  FAA;
(e) Within 9.26 kilometers (5 nautical miles) of any airport boundary
  without prior authorization from the FAA;
(f) In controlled airspace without prior authorization from the FAA;
(g) Unless you observe the greater of the following separation
  distances from any person or property that is not associated with the
  operations:
(1) Not less than one-quarter the maximum expected altitude;
(2) 457 meters (1,500 ft.);
(h) Unless a person at least eighteen years old is present, is charged
  with ensuring the safety of the operation, and has final approval
  authority for initiating high-power rocket flight; and
(i) Unless reasonable precautions are provided to report and control a
  fire caused by rocket activities.
§101.27   ATC notification for all launches.
No person may operate an unmanned rocket other than a Class 1—Model
  Rocket unless that person gives the following information to the FAA
  ATC facility nearest to the place of intended operation no less than
  24 hours before and no more than three days before beginning the
  operation:
...
§101.29   Information requirements..
...

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text:
I've been denied VFR clearance through C airspace, what should I do?
If I'm flying along in my C-152 and I am trying to go in a straight line through the "center" of C airspace (to land at a regional airport under the airspace), but the controller tells me to remain clear, what should I do?
Would it be safe to fly under the C, or above it, or would it be best to completely circle around even if it takes extra time and fuel?

Technically speaking a clearance is not required for class C airspace. All that's needed is to establish and maintain two-way communications. Most class C ceilings are only 4000 feet so flying over is pretty easy.

If I'm flying along in my C-152 and I am trying to go in a straight line through the "center" of C airspace, but the controller tells me to remain clear, what should I do?

Well, the obvious answer is remain clear of the Class C airspace :-)
HOW you remain clear is entirely up to you - you can go over, under, or around - you just can't go through, and I think that's the more interesting part of your question.

Flying around the class C airspace is usually a safe choice (don't skim along the margins though - give it some reasonable breathing room), but as you've noted it could add a lot of time/fuel. You need to balance that with the time/fuel required to climb above the airspace, or the risks of flying under the airspace (mainly obstacles) to make your decision.
I'm going to use my local class C airspace as an example for an exercise in aeronautical decision making because conveniently there's a VOR you could hypothetically be circling while talking to approach, and an airport on the other side you might want to get to.
In our hypothetical example let's say you're circling Deer Park VOR (A) at 2500 feet, and you want to cut through the Class C airspace to get to Brookhaven (B), but approach isn't cooperating.

Click to embiggen.
I've got three options sketched out here - think about them, then hover over the spoiler boxes for my thinking on each of the three routes:

The dashed line through the center (the route you were hoping Approach would give you).

 A straight line is the shortest distance between two points, but this straight line stinks: Since Approach wouldn't let you go through the class C airspace you're going to have to spiral-climb to 4200 feet or higher (5500 feet if you're playing by the hemispheric rule), and then when you get to the cut-out in the class C on the other side you're going to have to dive down to get to pattern altitude.
That doesn't seem safe, nor efficient to me, so let's toss that idea out the window.

The northern route under the class C airspace.

 Simple enough, we'll split the difference between the ground (roughly sea level) and the floor of the airspace (1500 feet) and fly under the shelf at 1250 feet. Plenty of room above and below (if your altimeter is set right and you're any kind of decent pilot).
 So this is the part where I confess that I'm a big ol' wuss and don't like to be close to the ground. It's like a porcupine, but with radio towers for quills (like the one I marked with a big OUCH - at 1250 feet there'd be less than 500 feet between my landing gear and the top of that tower).
 I'm not happy with the obstacle clearance on this route (and while I could certainly rearrange the line a bit to avoid the obstacles I'm not going to because they serve my purpose) -- the northern route is out.

The southern route under the class C airspace.

 Same idea as the northern route - descend to 1250 feet and fly under the shelf, but I like this route a whole lot better.
 There are really great landmarks here ("Fly southeast toward the looped road, then go to the middle of the bay and stay between the two shore lines until you pass the bridge at Smith Point. At that point you'll see the airport off your left wing and can maneuver to enter the pattern.")
 You're also over water, and we don't tend to have any radio towers sticking up out of the bay.
 You'll need to watch out for traffic from Bayport, students practicing ground-reference maneuvers, and banner-tow planes in the summer, but that's just your basic everyday VFR "See and Avoid" situation.
 This is probably the route I'd pick.

Incidentally I think this might be the first time I've legitimately used the >! markdown spoiler tag.

The only thing that I would do differently would be to fly at 500' below the shelf instead of "splitting the difference".

@Lnafziger You can definitely get by at 1000' on the southern route over the water but over land you'd technically be in violation of 91.119(b) a lot of the time since all the suburban sprawl & houses are above 0' MSL & a lot of the area is considered "congested".

+1 This is a great use of a puzzle-within-an-answer approach - very nice! I think I'd rather the northern route, as there are more options for landing if something goes wrong over the water (Class C radius is 5 NM, and engine out in 152 won't make more than ~2 miles @ 1200ft). I'd also rather fly just below the shelf ceiling - perhaps 150' below.

@CJBS Emergency landing options are a great point! At the risk of too much of a spoiler, your landing options in both routes are kinda lousy (check out a Google satellite view). My personal preference is to add a few minutes and get out from under the class C entirely (and in winter, a further preference for the south shore as the barrier island has nice, straight, runway-like roads which tend to be empty in the off season). This example is obviously highly contrived, and ignores such real-world considerations as good emergency landing options :-)

@voretaq7 and hopefully the last time you use the >! markdown!

My choice would be option 4:  It adds just a small amount of distance to option 3.  Continue southerly to the ocean, fly eastbound along the beach outside the Class C airspace.  The shoreline provides a unmistakeable reference to where the C ends.  You will have a great view.  The beach is a long emergency landing field (less crowded in the winter than in the summer), and you are not restricted as to altitude.

When the controller tells you to "remain clear" it simply means that he is too busy to help you out at the moment.  Traffic situations change quickly though, so I would recommend taking the most direct route that you can while remaining outside of his airspace and when things sound like they have quieted down a little, give him another try.  If it isn't feasible (you are landing at the primary airport, or you need to enter it to get to the other airport) then you can circle until things calm down for the controller.
Since the airport that you are wanting to land at is under the Class C airspace anyway, probably the best way would be to go ahead and descend a little earlier than you had planned and fly under the shelf.  Assuming that you have sufficient terrain clearance it would probably be better than flying over the top since you have to go down there anyway.  That being said, there's no reason not to fly over the top if it's more convenient!
In either case, the main thing to watch for is arrival and departure traffic for the primary airport.  The controller basically told you that there is a lot of it.  If you are instrument rated, take a look at the arrival and departure procedures so that you can try to avoid them.
Just be sure that you don't accidentally fly into his airspace without a clearance and you should be fine!

As indicated in the other answers, "remain clear" means just that, and can happen when congested (as already indicated by @Lnafziger).
One approach that I take in the Bay Area to have a greater chance of getting into Oakland's Class C or SFO's Class B is to get a squawk code (with a flight following request) from another facility first, then get handed over. That way, it's just a matter of a frequency change and a hand-over. E.g. "N12345Z on frequency at 2000".
State to the first facility (squawk code assigner) what your intentions are so that they can give you the correct squawk code (they are different sometimes depending on whether you need to be handed off to another facility)

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text:
What options do pilots and mechanics have when an aircraft owner doesn't pay up?
If someone does a contract flight, or a mechanic does contract maintenance for an aircraft owner and they refuse to pay after the fact, what options do they have to "encourage" the owner to pay?
One of the big problems is that this tends to happen informally:

Owner: "Hey, can you do this flight for me?"
Pilot: "Sure, I charge $xxx." "
Owner: "Okay, great.  See you on Monday morning at 9:00."
Pilot: "Sounds good, see you then."

So they show up, do the flight, send an invoice, and wait.  And wait..  And wait....  After a few weeks or 30 days, they send the invoice again.  Give it a bit and make a phone call.  Eventually it becomes clear that they just aren't that interested in paying a lone individual without the typical resources of a larger company.  Unfortunately, this happens more than you would think in this industry.
It probably isn't enough money to actually go out and hire a lawyer for, but it's irritating and they want their money.  It's the principle!  I've heard of people placing a lien against the aircraft.  Is that something that an individual can do, or do you have to hire a lawyer?  Is small claims court an option?
I'm looking for an answer that describes general approaches that can be taken, as obviously specific legal advice cannot be given without details of a situation like this.

I'd say that Small Claims Court sounds appropriate for this.  Small Claims Court generally prohibits lawyers on both sides, and limits awards to about $2,500 or less.  Cases are heard quickly, without a lot of legalese, and the judge's ruling is final and binding. (details may vary state to state, so check your state)

Is this really about aviation? In what way might the answer be different for an aviation firm compared with a limousine service, or a plumber?

@DJClayworth There are aviation specific aspects to this question such as how to register a lien with the FAA, working with different states (where the aircraft is registered -vs- operated), etc.  In any case, even if it wasn't specific to aviation, it is an issue that those in aviation have to deal with.

@Lnafziger the legal recourse you have is the same as any other contractor or employee who gets faced with non-payment by their customer or employer. The only thing different is where you file the paperwork.

I think this question would work better on law.SE. It's fundamentally about breach of contract, not aviation, and in any case it may be too broad to answer because any answer probably depends on the state in question.

@Pondlife Perhaps, but as far as the sate, which state?  What if the owner has houses in multiple states, aircraft is registered where it is most beneficial, the pilot lives in another, and the flight took place in (possibly several) other states and/or countries?  I think that there are some common situations in aviation which can be answered in a general way without just saying "go ask a lawyer" (or law.SE, as the case may be, lol).

You should be able record a lien against the aircraft.
Avionics News ran a Legal Ease piece about just this, which is well worth reading fully:

If a lien is properly asserted, the lien claimant (the aircraft mechanic) has encumbered the aircraft in question, meaning the aircraft no longer has clear title.
Clear title is highly desirable to an aircraft purchaser, who can determine if an aircraft has any liens on its title through a simple title search. Once there is a lien on an aircraft, to sell the aircraft with clear title, the aircraft owner must obtain a release from the lien claimant. This means the owner must pay you for your services before they are able to sell the aircraft with clear title.
If the owner fails to pay the lien claimant to have the lien lifted, the aircraft will be sold subject to the lien. This means the aircraft does not have clear title, and although the aircraft will have a new owner, your lien will remain in effect.
Most buyers, however, will not purchase an aircraft subject to a lien, and thus, you usually will manage to receive the money owed to you when the owner decides to sell the aircraft.
If you do not want to wait for the aircraft to be sold to satisfy your lien, you can bring a foreclosure proceeding, forcing the sale of the aircraft, usually at an auction.
The proceeds from this sale go to the lien claimant to extinguish the lien. If the proceeds are greater than the lien, any money exceeding the amount of the lien goes to the aircraft owner. Additionally, if there are no bidders at the sale (an unusual occurrence), the aircraft may be given to the lien claimant for the price of the lien.

One thing worth bearing in mind, is the Aero Liens List - a free resource which lists liens placed against aircraft owners. If your new client is on their "deadbeat client list" then you may want to think twice about doing the trip or get paid in advance!
You asked whether a lien could be registered by a pilot for lost wages (rather than a mechanic for lost labour/parts); just searching the above list for "John", I found at least one pilot who has two liens (against seperate companies) for lost wages: #1 #2
Another lien (PDF unavailable) lists its nature of the claim as "Salary owed".

The problem with liens is that they only really matter when the plane is sold.  If the owner has no intention of selling (and many plane owners keep their planes for decades, often dying while still owning it), the lien is worthless.

@abelenky As mentioned in the quote, bringing forward foreclosure proceedings would force a sale. Whether the salary owed would be high enough to allow a foreclosure, I don't know.

@abelenky: If a plane owner dies with an outstanding lien on their aircraft, would that have implications for inheritance purposes?

It depends on your state laws for what is permitted for filing a Mechanics Lien. For example in California you cannot hold the persons property. In Texas, you can both hold the property and then sell it at auction if they don't pay up.
If you would like to file a Mechanics lien, the cost is $45 dollars and can be processed through www.planefax.com which operates directly within the FAA's documents room in Oklahoma City. Plane Fax can also further advise if you have a valid lien or not. The process is called perfecting a lien.

There are two avenues available to mechanics (but not pilots).

1) Apply a mechanic's lien. As pointed out by Danny Beckett
  and Jason many States allow the property to be held until payment is made.
  The exact authority and privileges vary State by State.
2) All shops I worked for had strict instructions for the mechanics to
  not sign the logbook until payment is made. This keeps the aircraft un-airworthy until payment is made.

A pilot does not have either of the above two options, their only recourse is the court system.

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text:
Will a portable ADS-B unit, like Stratus, meet the requirements for my aircraft to be ADS-B equipped by the 2020 mandate?
There is a lot of confusion around what it means to have ADS-B.  What is the minimum equipment required to meet the mandate? Can I use a Stratus to meet that requirement?

The short version is no the Stratus will not meet the requirements, and for several reasons.
The final rule requires ADS-B OUT (and the Stratus is only ADS-B IN) that meets particular TSO requirements (see below), and portable units will not.
The FAA has an ADS-B FAQ on their website which has a lot of great ADS-B information.
The minimum equipment required to meet the mandate is addressed by:

What equipment is required by the new rule?
The rule specifies ADS-B
  Out equipment compliant with either Technical Standard Order
  (TSO)-C154c (Universal Access Transceiver) or TSO-C166b (1090 ES).
  However, to operate in Class A airspace, aircraft are required to
  equip with avionics certified to TSO-C166b.

The FAQ also specifically addresses the question of portable units and says that they will not be allowed:

Will the FAA allow portable/handheld units, or will they have to be panel mounts only?
Installed transponders and GPS units must meet certification
  standards. Handheld devices and displays that serve only for
  situational awareness have more flexibility and are not certified
  installations. In order to comply with 14 CFR § 91.225 and 91.227
  aircraft intended to fly in ADS-B airspace must have installed and
  certified equipment. Portable installations are not compliant to the
  rule and would only be usable for receiving FIS-B services for
  situational awareness. The FAA is in the early stages of investigating
  a portable device for glider aircraft.

"Portable installations are not compliant to the rule" is probably the single most important part of that second quoted section.

The SkyGuardTWX Vision-Pro ADS-B Transceiver is one such portable device that can provide ADS-B out, which is planned to meet FAA 2020 ADS-B OUT requirement once TSO-C154c certification has been completed.  However, it has not yet been approved.  If it is eventually approved, there might be more portables from other vendors as well.

If I remember right, one of the requirements is that the ADS-B out has to be individual aircraft specific. So, that negates the portable ability.

Each US registered aircraft is assigned a unique mode-s identification.  This id must be programmed into the ADS-B transmitter.  A portable unit can have that information configured for it.  I would be aghast to learn that the portable transmitters did not have a menu option to toggle between several aircraft.


text:
How difficult is it to avoid ground loop in practice?
In my simulator (flightgear) directional control during take-off of most tail-draggers is extremely difficult. When the tail lifts off, it immediately starts to turn to the left with no crosswind, but with sufficiently strong crosswind from the right it might be to the right. It takes very swift and hard rudder application to arrest and then easing the pressure to avoid overcontrolling. Otherwise the turn rate keeps increasing and ground loop occurs.
It is worse with some models than others. Some models are almost impossible to take off at full or normal take-off power and while most are, it is very difficult to keep them on the runway, because even if I manage to arrest the initial swerve, the aircraft is then turned a bit and attempts to re-align it tend to again provoke ground loop.
Controlling propeller aircraft with tricycle landing gear is on the other hand never difficult, even for the more powerful ones.
I suppose some of the models are wrong. For example the Pitts S1C model is almost impossible to take off. But how difficult should it really be to keep the plane aligned?
And, what would likely be wrong with model that is too difficult to control? Centre of gravity too far aft of the mail landing gear? Or something with the vertical stabilizer? There does not appear to be any parameter for vorticity of the propeller slip stream; it is apparently calculated from the power.
Or is this one of the cases where simulator is much more difficult than the real thing, because in the real thing you have force feedback and get to use your built-in accelerometer (vestibular system) and in the simulator you don't?

What you're experiencing is the natural tendency of an aircraft to turn left under high power.
Several forces combine to cause this as shown in the diagram below:

(These forces are also discussed in this YouTube video which is worth a watch.)
The left-turning tendencies are more pronounced in higher powered aircraft (more torque). They are even more pronounced in tailwheel aircraft when the tail comes up because you no longer have a third point of contact with the ground resisting the forces, and the pitch change exaggerates the effect of gyroscopic precession. (Similarly when the tail comes up in a strong crosswind the aircraft has an even higher tendency to weathervane into the wind, because there's no third point of contact with the ground fighting its tendency to pivot).
The position of the center of gravity relative to the gear also contributes to some of the differences in handling on the ground (and their tendency to ground-loop), but it's probably not the major factor in the difficulty you're describing.

In regard to your difficulty fighting the aircraft's left-turning tendency, part of this is undoubtedly the fact that a simulator has poor (zero) feedback compared to sitting in an actual aircraft, but some of it probably has to do with your technique: You're describing very swift and hard rudder application, and frankly no aircraft is going to like that (and tailwheel aircraft will make their displeasure known in the swiftest possible ground-loop).
What's called for instead are smooth control inputs - advance the throttle slowly, feeding in enough rudder input to track the centerline as the engine power increases. Remember that ultimately you're balancing forces that are changing: Slipstream and torque reaction are more pronounced as the engine develops more power,  P-factor varies depending on the aircraft's pitch angle, and the effects of precession vary as you change the aircraft's pitch angle (applying forces to the rotating propeller).
While these forces are changing the force exerted by the rudder is also changing, becoming more effective as you accelerate.
Because of the way these factors combine the corrective forces you need to apply with the rudder aren't constant, and your feet should be "dancing" on the pedals making small corrections all the time as smoothly as you can (a little wiggle on the centerline is far preferable to stabbing at the pedals and making the aircraft pirouette on the runway).

Since you're using a simulator you can practice all of this in a no-wind situation to get a feel for the aircraft and the sort of control inputs that will be required (and ideally you can start with a trainer aircraft like a J-3 cub, and move up to something with a bigger engine as you get comfortable). You can also do high-speed taxi runs (with the tail up) to get a good feeling for how everything interacts, without having to worry about breaking a perfectly good aircraft if you ground-loop.

Disclaimer: I've got all of 1 hour in a tailwheel aircraft and the best I can say for my advice above is "by following that advice I didn't ground-loop the thing!"
  You can probably get far better advice from a CFI with tailwheel experience, or a tailwheel pilot who's done it far more (and probably far better) than me!

You say apply rudder progressively as increasing power and while the tail wheel is still on the ground. Well, that might be the problem with the simulator. Because while the tail is on the ground, the simulated aircraft tends to go straight with very little effect of the rudder and then it starts turning very suddenly when the tail lifts off.

@JanHudec The left-turning as the tail comes up is almost entirely gyroscopic precession like Peter mentioned (and maybe losing the tailwheel, if it's a steerable wheel) - it sounds like your simulator is doing the right thing. Basically as the tail comes up you'll need more rudder pressure, and when you stop changing the pitch attitude of the plane you'll need less - how much more/less depends on the prop size / speed which determines the gyroscopic rigidity the prop experiences.

Thanks. The steerable wheel should be loosing grip progressively and therefore it's effect should cease progressively, but as long as gyroscopic precession is indeed the important effect here, it makes sense. And maybe suggests that applying some up elevator to make the tail come up more slowly should help.

Your simulator gives you a realistic experience: This is normal. What you experience is the gyro effect of the propeller and engine while you change the pitch attitude. When you lift the tail, the axis of rotation is changed and the engine-propeller combination pushed downwards, causing a precession force which acts orthogonally to the plane defined by the down movement and the axis of rotation.
Since the engine is forward of the center of gravity, this precession force is pushing the nose sideways: Left for a propeller which spins clockwise (when observed from the pilot's viewpoint), and vice versa.
The best way to avoid a change in direction is to keep the tail to the ground with the stick aft until you know you are fast enough to lift the tail, and when you lift the tail up, you apply rudder simultaneously. Don't let the tail come up by itself!
I learned this in a Dimona motorglider. Now think of flying a small, light airplane where the propeller is bolted firmly to the heavy engine crankcase, both rotating. The airplane will dive or climb when you change direction! This is what pilots had to endure in the First World War, when they flew aircraft with rotary engines. When engines became more powerful, this became so bad that some aircraft were almost unflyable. See this answer for more.
That crosswind is turning the nose into the wind after lifting the tail should also be no surprise: While the main gear is still on the ground, the plane will weathervane into the wind once the tail is lifted.

What about the tendency of the turn to develop into ground loop rather rapidly if it is not corrected very swiftly with rudder? The effect of course also exists. But how bad is it in real, properly balanced aircraft?

@Jan Hudec: My experience is limited to planes with large moments of inertia around the vertical axis (big wingspan!) and small propellers, so they never showed more than maybe 10° deviation from the runway line. On smaller, more powerful engines the side force would be bigger and cause more directional change. And then you are moving sideways with the main gear down, which could very well result in a ground loop.

@JanHudec My first solo was in a tail-dragger and I did a ground loop on landing. The problem with that type was that the tailwheel was controlled by the rudder pedals for deflections of maximum 28 degrees (the same as the rudder). After that, the tailwheel disconnected from the controls and it behave like the wheels of a shopping cart, usually increasing its deflection. So, if the pilot didn't control the turn early on, the situation developed rapidly to the point where there was no control on the wheel and due to low airspeed, no control on the rudder.

@Emil: On landing, the propeller spins less quickly, so precession should not be a factor. I am at a loss to explain what caused your ground loop.

@PeterKämpf: Yes, the deviation was not as important as during take-off, but it was visible. Especially after a touch-down with the tail quite high, when the tail started to drop, there was a significant deviation. If you add here poor visibility on ground and little flying experience, going 30 degrees off-track was a matter of seconds.

@Emil: Interesting observation, thank you for sharing it! I think you were still quite fast on touchdown, because normally a taildragger is landed such that little tail drop is needed. Precession is not depending on thrust, just the rotational energy of the prop. It seems you dropped the tail quickly, so even at idle RPM the propeller produced a significant precession. Also, your plane probably lacked the big inertia of glassfiber motorglider wings, so the effect was much heavier. When it happens, it happens quickly, and when you are not prepared, you can lose control easily.

I have a few hundred hours of taildrsgger time, both airplanes and gliders. Even though gliders don't have an engine they have to be kept flying straight especially when the tow rope hook is under the CG instead of the nose.
There are two things that matter, practically speaking:

Don't let the tail wheel come up until the rudder is moving fast enough through the air to have full effectiveness. Many TWs have locking TWs to assist with keeping the airplane straight
anticipate the yawning, be aggressive when correcting, and "uncorrect" to keep the nose  straight. This means that when you bring up the tail, start feeding in rudder, faster than you need it, and before the nose is straightened out, start feeding in opposite rudder to prevent from overshooting. If you overshoot, you will start to oscillate with larger and larger movements, and you will lose control

I have not flown that many taildraggers, but some can be kind of twitchy like you describe.
Like most things in flying the trick is to not get behind.  If you anticipate needing some rudder as you feed in power get the control input in before the power.  Also, remember as the aircraft picks up speed the control surfaces will be more effective, so you need to moderate your control inputs.
You also have to compensate for the descending prop blade causing the center of thrust to shift off center because of the high angle of attack of the prop blade as you start rolling.  This commonly called p-factor.
Each plane, let alone model of plane, tends to be unique in it's handling characteristics. While I have never flown a Pitts I can imagine that since it has a lot power (big p-factor), short fuselage (short moment, lower longitudinal stability), and rather big control surfaces (small input, big effect) it would be a handful.  Maybe you should try something easier for a bit like a Piper and get that down.  Then work up to a stable plane like a Cessna 185, and a twitchier plane like a Citabria, and then onto the Pitts.
Also, I think the Pitts does not have flaps, so you will probably need to slip to a landing.  That is really fun and it also provided good runway visibility since you are coming in at an acute angle to the runway.
Have fun,
Jerry

I have 5,000+ hours of tail-wheel experience. So, heres my two cents:
I won't elaborate on the physics of what's happening since voretaq7 explained that topic pretty well (thankfully). I will concentrate on a more practical understanding.
How difficult is it to avoid ground loop in practice?
It's not difficult, you just keep the plane straight...at all times, no matter what it takes. I would say your simulator isn't very accurate in portraying real-life tail-dragger characteristics. I have never flown a Pitts, however of the other tail-draggers I have flown, they are not extremely difficult to keep straight, just attention.
what would likely be wrong with model that is too difficult to control?
One where the aircraft does not have enough rudder-authority to compensate for either:

The current wind velocity.
The amount of torque the engine produces.
A combination of 1 and 2.

These characteristics seem to be more per-dominate by the design of an aircraft as well. (short-coupled and narrow-wheelbase aircraft being in that group). I do not know much about physics, so I can't elaborate on the why, other than I know from experience.
I find that once the tail is up and flying, it is EASIER to keep the airplane straight, you have more rudder authority (i.e. control) of the aircraft to main directional control, but that may be just me.
In short, flying a tail-dragger is not all that difficult, it sounds like your simulator does a poor-job giving tail-dragger flight characteristics. With that said, I also find simulators to feel very un-natural.
As far as flying a real tail-dragger. It takes a bit getting used, but it's like anything else. Once you get the hang of it, your good to go.

Thanks. I suspect a big problem with simulators is that they only give you visual information and no associated feel of motion and not correct feedback on control (unless you buy a special force-feedback joystick that I don't have). And even the visual is not really that good; when I learned how to handle skid in a car, I was told to keep my head aligned with the road facing external reference (and it did help a lot), which does not really work in simulator.

You said "just keep the plane straight... at all times, no matter what it takes".  However, some tail-wheel airplanes sit on the ground with such an extreme "nose-up" stance, the pilot can't see a damn thing in the forward direction.  So how does a pilot keep the airplane pointing in the direction of the runway center line... when he can't see the runway?  In some cases the pilot can't see anything at all forward, not even trees in the distance, so he can't replace a view of the center-line with a view of a tree straight ahead.  Seems like looking sideways out the window wouldn't help much.

How difficult is it to avoid ground loop in practice?
Not difficult. Practice makes perfect.  That's why in real world type ratings exist, you get extra training to fly taildragger types.
Desktop PC simulators are a waste of time and not at all realistic, the "full-motion" ones airlines use are lightyears better... but obviously nothing beats the real thing !


text:
How do you calculate minimum runway lengths for Part 135 operations using the 60% rule?
FAR 135.385 requires that at your destination airport your flight planning shows that you could make a full-stop landing "within 60 percent of the effective length" of the runway when flying transport-category turbine-powered aircraft.
For piston-powered aircraft FAR 135.377 has a similar 70% rule.
How do you calculate minimum runway lengths under these rules?
Can you use 80% of the runway? What if the runway is wet?

I've added a bit to your question to point out the specific parts of the FARs I think you're asking about -- If I mangled it feel free to revert my edit :)

Required runway length for landing
The minimum runway length required for landing under Part 135 operations is, believe it or not, the same as Part 91: You need to be able to land and stop within the available runway length specified in the AFM.   This is what you use when you are getting ready to land.
That being said, since you reference the "60%/80% rules", there is a more stringent requirement that is required as part of your preflight planning and specified in 14 CFR 135.385
60% Rule for Large Turbine Transport Category Airplanes
This rule is specified in 14 CFR 135.385(b), and says (in part):

no person operating a turbine engine powered large transport category airplane may take off that airplane unless its weight on arrival ... would allow a full stop landing at the intended destination airport within 60 percent of the effective length of each runway

So to follow the regulation by the letter, you need to take 60% of the runway length and compare it to your actual landing distance.
Example
We want to land on a 6,000 ft. runway, so our max allowable landing distance is 3,600 ft.:
$$6,000ft\times60\%=3,600ft$$
Determining minimum runway length
Sometimes pilots prefer to ask the question in a different way, and ask "what is the minimum runway distance needed for the current conditions" instead of "what can my maximum landing distance be.  To calculate this, we simply re-arrange our formula above like this:
$$Minimum~Runway=\frac{Actual~Landing~Distance}{60\%}$$
If our actual landing distance is 3,000 ft. then we would need a minimum field length of 5,000 ft.:
$$\frac{3,000ft}{60\%}=5,000ft$$
80% Rule
The 80% rule specified in 14 CFR 135.385(f) works exactly the same way, but can only be used by an "An eligible on-demand operator" if approved in accordance with their Operations Specifications (A057 and C049) and the procedures listed in their Flight Operations Manual.
You would then replace the 60% with 80% in the above calculations.
Wet Runways
14 CFR 135.385(d) says:

(d) Unless, based on a showing of actual operating landing techniques on wet runways, a shorter landing distance (but never less than that required by paragraph (b) of this section) has been approved for a specific type and model airplane and included in the Airplane Flight Manual, no person may take off a turbojet airplane when the appropriate weather reports or forecasts, or any combination of them, indicate that the runways at the destination airport may be wet or slippery at the estimated time of arrival unless the effective runway length at the destination airport is at least 115 percent of the runway length required under paragraph (b) of this section.

So basically, you use the information from your AFM for wet runway landing distance, or if it isn't provided (since it isn't required), you do the same calculation that you did before but include an additional 15%.
Example:
Taking the example from the 60% rule and assuming that it is wet, we have a max landing distance of 3,130 ft.:
$$\frac{6,000ft\times60\%}{115\%}=3,130ft$$
Minimum Runway Length When Wet
The minimum runway distance with a 3,000 ft. actual landing distance would be 5,750 ft.:
$$\frac{3,000 ft\times115\%}{60\%}=5,750ft$$

Simply put: your description of the 60% is WRONG!
Part 135.385b does not speak to geography, it speaks to performance!
"Effective length of runway" is not a percentage of real estate, but actually a performance number.
The airplane performance section of the AFM will provide the landing distance given the applicable condition(s). It is this number that derives the "effective runway landing distance."
As the regulation stipulates (without going into: zero wind, expected wind, or anticipated wind), the performance number is divided by .60 for 60 percent, or .80 for 80 percent.
Remember, the effective length of the runway is measured against a point where the approach path clearance plane (APCP) intersect the threshold to the end of the runway.
Since the APCP is a 20:1 slope flown at 1.3 VSO, and flown 50 feet above the APCP, the math is simple: 20x50=1,000 feet.
The 1,000 feet is where the wheels are expected to touchdown and is accounted for when you calculate the distance by dividing the AFM landing distance by 60 or 80 percent. The effective runway landing distance is within the entire length of the runway, not 60% or 80% and where the APCP intersects the threshold to the published end of the runway. If the runway is displaced, the displacement is accounted for in the effective runway distance.
While your reported procedure is close to accurate, it is not the proper method for determining Parts 135.385b or 135.377.

I can't decide if this is an answer to the original question or an attempted response to Lnafziger's answer.


text:
When an airport has helipads, why would a helicopter take-off or land on a runway?
At an airport with both runway and helipad (KASH) I have seen both being used for helicopter takeoffs and landings. Is there anything that causes the helicopter pilot to choose to use the runway instead of the helipad?

I know you asked specifically about runway versus helipad departures, but I thought I would lay out all the options available to the heli pilot, and you can see what all the choices are.
There are three different areas on an airport that a helicopter can operate from:

Non-movement area -- this is the area of the airport that is not controlled by the ground or tower controller. It often consists of aircraft parking areas or an area leased by a tenant.  It is separated from a movement area by this the marking below. Helipads may be found in non-movement areas, such as the Coast Guard helipads at SFO and JFK.
Movement area -- this is the area of the airport controlled by a ground controller, and separated from the runway area by this marking: . Helipads can be found in movement areas.
Runway area -- this is the area controlled by the tower controller, and contains the runways used for landing and departing traffic.

This picture from KHWD shows the three areas:

Regardless of which area is used, almost heli departures from towered airports are coordinated directly with the tower controller because the aircraft enters the controllers airspace.
At non-towered airports, helis operate out of the flow of fixed-wing traffic, and need to self-announce like any other aircraft.
Additionally, helicopters can make three different kinds of departures:

max performance take off -- which begins with a near-vertical segment, and then transitions to a horizontal. This is typically used from a confined area, and looks something like this:
normal takeoff -- this begins in an in-ground effect hover with a horizontal segment until translation lift is assured, then transitions into a climb:
running takeoff -- when there is insufficient power to begin with an in-ground effect hover, then the helicopter may begin its departure by sliding (skids) or rolling (wheels) along the ground, until translational lift can be obtained, then transitioning into a normal takeoff:

The choice of departure area and departure type are for the most part independent, but they are obviously linked based on performance and obstacles.
The departure area is an operational decision. When the helicopter is operating from a remote part of an airport, the pilot will often make a departure from a non-movement area (helipad or ramp) so she doesn't have to taxi to the runway.  If the helicopter is a passenger flight, such as air-taxi, it will most likely operate from the runway or a helipad in the movement area.
The departure type is a performance decision, based on obstacles, weather, weight, and available power. Hot & high departures might require a running take-off. Lightly loaded twin-engine copters in normal weather can safely make high-performance departures over obstacles in the desired direction of flight.

Spot on.  This sentence is the crux of the answer - The departure area is an operational decision

i have noticed that helis with wheels like to do running/rolling departures on runways. Also, I have seen a lot of large military helis depart in formation using rolling takeoffs from different parts of the runway, in order.

Yeh.  I think that's because of wear on the skids, especially from runways.  If I had wheels, I guess I'd do a rolling take off every time since it's the lowest power requirement.  Even on grass, the skid plates wear.  But as you say, ops is the key.  I worked on a line of Wessex IIs (H34 to the cousins) and the layout of the pan was such that hover take-off followed by hover taxi to a dedicated grass runway was the norm.

I have heard helicopter pilots being instructed to land either on the paved runway or at least sixty yards away in order to avoid blowing foreign objects and debris onto it.

One reason is to avoid damaging light aircraft or other property
I had a case back in the 1980's where I had brought a Navy helicopter to an airshow for a static display. (Gross weight about 12,500 lbs).  While doing an air taxi to the spot where I was to display the aircraft, the tower told me to land immediately.  My rotor wash was causing some light aircraft that I didn't see clearly (downwind from us) to strain at their tie downs.  We used a rolling ground taxi to get to our position.  (Air show was good fun, good weather, etc).
When we departed, I coordinated with the tower to do progressive taxi to the runway, and to depart from there, because most of the aircraft from the airshow were still there, and we had to get back to our base.  We wanted to reduce risk as far as possible to anyone's light aircraft (there were a lot of them) from our rotor wash.
We took off from the runway to prevent damage/disturbance to other people's property at the airport.  All's well that ends well.

My impression was that runway takeoffs are easier / safer because you remain less in the unsafe zone where auto-rotation is precluded.
e.g. See the chart below. You want to avoid the red zone, the so called "coffin corners" (not to be confused with a similarly called but totally different operating envelope for Aircraft at high speeds). So airspeed helps.

Basically auto-rotation is your safety pass in case of engine / transmission failure etc. So you want to avoid situations where you are gaining pure height without any airspeed.
See the recommended take off profile on the chart below. It increases airspeed a lot before the height increases significantly. That's what you get in a runway takeoff but I guess could also do in a helipad takeoff depending on the surrounding constraints on flight path.

There is no difference in the take-off or landing profile to a runway or a helipad.  Remember helicopters do not (except in a few scenarios) roll along the runway and rotate. They lift into a hover, accelerate in ground effect, gaining translational lift until about 45 kts, then climb.  Same for helipads or runways.  I can avoid the red areas landing on a helipad just as easily as I can landing on a runway.

@Simon Right, but if you take the recommended take off profile you need to be doing close to 100 kmph at about 10 ft above the ground. The runway is one clearly safe place you can do this. If your helipad surroundings offer the clear area to do it, that's great too. That's why I had the part to my answer which clarified that you could get the same safe flight profile from a Helipad too but obstacle clearance is a must.

there are plenty of areas on an airport besides runways where normal departures can be done in the non-shaded areas of the H/V diagram, including taxiways and the grass between the runways/taxiways. I have even "landed" and "taken off" from Frazier Lake's water runway in a copter equipped with emergency pop-out floats.

Picture: http://frazierlake.com/pics/1C9/FLA_100-Over-Southeast-FLA-boundary-in-2009-by-Curtis-Fowles.jpg and map https://goo.gl/maps/CvwxwAAQ3y32

@curious_cat Where did you get 100kph from? Normal ops would never get to that speed before climbing. In most types, climb begins at 40-50 kts depending on density, weight and area profile. rbps answer is correct.  Choosing a runway or a pad has nothing to do with the possibility of having to autorotate,

@simon Isn't 50 knots 92 kmph?

I've never seen a helipad in real life that has any room to do anything other than a max performance take off (or landing along roughly the same profile) but this may be because all of the helipads I have seen have been at places like hospitals...

Though I'm not sure of the exact circumstances, the helicopters may be carrying out a rolling take-off. This is done in some cases where sufficient power is not available for normal (hovering) takeoff, like high gross weight etc.
In these cases, the helicopter is rolled down the runway to gain sufficient speed (to generate the required lift), so as to satisfy the power requirement. As this technique uses up considerable distance (for the takeoff roll and initial climb), and requires a smooth surface (during roll), the runway is preferred.

In case the helicopter had taken off from runway normally (i.e. in hover), the ATC might have asked the pilot to use the runway so as to let the other aircraft know which direction the helicopter is going.

you can make any kind of departure from a runway, but you can only make a hover (normal) or high-performance takeoff from a helipad

I've made many "limited power" take-offs from somewhere except a runway, including sliding skids across the ground on grass (which is actually preferable since if causes less wear on the skid plates, although there is a slightly increased risk of mistakes since on grass, a skid can catch on something).


text:
What is the wording on outline of Wright Flyer on FAA pilot license?
I was told once that the outline of the Wright Flyer was taken from a speech given by the Wright Brothers.  In the picture below you can very clearly see the words on the Wright Flyer.  Does anybody know what that speech was?

Picture taken from Wikipedia Commons

Picture taken from Keven's Aviation Adventure

Due to this question, I also learned from someone that the flip side of this type of certificate also has micro print on the shirt collars of the Wright brothers and the depiction of the Wright Flyer on that side. Perhaps you should edit this question to encompass all of that too.  I'd ask it, but I think it's better to keep it together.  What does the flip-side say?

"Without question, flight is represented by countless heroes who have stirred greatness in others. It was a dream that had been waiting to be realized for thousands of years, and chased by countless visionaries, scholars and brave souls. But even as the mass of collected knowledge came to a crucial turning point, it was not until the ingenuity of the Wright Brothers that the puzzle of powered flight was solved. Discovery requires deep understanding, constant interpretation of knowledge and unbridled persistence."

It is part of a speech given by Marion Blakey on December 17, 2002 at the Air and Space Museum for the Centennial of Flight Kick-Off event. Unfortunately I can't find the full text of the speech (for free at least, it is part of a book you can buy for \$5 used, or \$108 new). Marion Blakey was the 15th FAA Administrator from 2002 to 2007.

What part of the plane are the words depicting?  I'm wanting to say stringers, but I know that's internal.  Would they be called struts?

The words make up the vertical (struts would be my guess) supports between the upper wing and the lower wing.


text:
What causes descending aircraft to make a noise like a giant blown bottle?
I've lived for years about eight miles NW of Boston's Logan Airport. Although I can't say we get a lot of aircraft noise, once in a while, due to winds or air traffic, there will be a long line of planes traveling right over my house, perhaps 6000-8000 feet up, descending into Logan. At night you can sometimes see three in a row, with a fourth one turning on its landing lights.
Most of the time the overhead planes sound mostly like a roar with some jet whine. However, once in a while a plane will suddenly start making a very strange sound like an enormous blown Coke bottle. Often the pitch will descend during a half dozen seconds, stay constant for a dozen seconds, and then rise again until the sound disappears.
My best guess is that the sound is caused by the pilot lowering the wheels during their descent, and that it only happens on certain models of aircraft. As the wheel well covers open, the air blowing across the wells sounds like that giant Coke bottle, with the pitch decreasing as the opening increases. On certain models, the covers will then close once the wheels are down, causing the pitch to go back up.
(Edit: it's been pointed out that gear isn't dropped until 2000-3000 feet, so in my case that isn't the sound. So, I've removed all those videos, cool though they may be.)
Is my guess right? What model/manufacturer of large jet would make this sound?

If it is at 6000-8000 feet, then it won't be the landing gear. It's probably first flap extension. During flaps transition some aircraft make funny noises.  If they are lined up for the runway when they are above your house, and the runway is eight miles away, they are about 2000-3000 feet heigh and it may be the gear.

You can find it out for yourself by using a flight tracking website like flightradar24. Just note the exact time that you hear the noise and correlate that to the flight tracks at the website. It shows the aircraft type.

@DeltaLima Planes can't be on final approach as they come over us, as closer towns would scream. They circle over the Atlantic as they descend.

@DeltaLima Can flight tracking websites give true real-time data? I remember when they had to be delayed 5 minutes for fear of assisting terrorists.

FR24 gives live data for the yellow planes, the orange planes are delayed by 5 minutes.

related question?

@Michael Thanks, but no. I hear the effect I'm trying to describe on the back of my house's porch just about every day; at some point I'll record it and return to this question.

@DanielGriscom Ah, ok.  It seemed similar... happens frequently if I'm outside when the flight pattern goes over my house, pitch goes up and down, etc.

@DanielGriscom Have you found that the FOPP hole answer below is likely what you're hearing? I'm a few miles out from O'Hare in a landing path and have struggled to describe the sound I am hearing, and "blowing across the top of a giant bottle" is a great approximation.

Unfortunately not. Every explanation I've seen means that, from the plane's viewpoint, the generated pitch would be constant; the pitch changes heard from the ground would be Doppler-shifted downwards as the plane passed. Unfortunately, the sounds I hear go down in pitch, and then go back up. And, I need to do a better recording of what I hear and then enhance this question; sorry to all who have visited this question over the years.

Short Answer:
The sound is caused by airflow across holes on the underside of the wings of certain aircraft.
Long Answer:
I too have noted the sound phenomenon that you describe. This video has a great example between 0:26-52 (link starts playback at 0:26 where the noise starts):

I will readily admit that this answer is mostly just a compilation of what I have learned researching this. Your question intrigued me and I've enjoyed learning about it!
As I discovered as I started researching this, many others are familiar with the noise as well. This article on Airliners.net had a lot of discussion about it, starting with a lot of interesting—but ultimately flawed—theories.
But, that thread led me to the answer to your question, which is: the sound is caused by the "circular opening used to equalise the pressure in wing fuel tanks", according to Euractiv.com
This article on AirportWatch further explains:

The Airbus 320 series of aircraft, [...] have been known for many years
[...] to have a particularly irritating high pitched
whine. This is caused by air rushing across the under-surface of the
wing, where there are Fuel Over Pressure Protector (FOPP) cavities.
This generates noise, in the same way as blowing air over the mouth of
a bottle. Every A320 series aircraft emits a signature howling noise
while approaching to land. It is heard most when the plane is
travelling at around 160 knots, and the frequency is around 500-600Hz,
which is close to peak sensitivity of the  human ear.

That being said, there is apparently a fix to that problem, as both the aforementioned links describe. As Euractiv.com explains, the fix is a vortex generator:

a 5-cm triangular piece of aluminium sheet metal upstream of the two
vents on each wing, in order to divert the air flow and stop the
whistle. A decade of research went into the vortex generator, which
stems from efforts to mask a similar whistle produced when air blows
over the gun ports of warplanes.

This image shows both the FOPP hole, and the VG that softens the sound:
[Image Courtesy of Lufthansa]
Read the articles linked, interesting stuff therein!

I think we just meet with the same curiosity I had. I hear that sound you describe also, and I already reduced the observation on it happening with Boeing 757-200's only. Like the ones operated by United. I happened to have found the answer, this is exact technical explanation.:
http://www.airliners.net/forum/viewtopic.php?t=445103
It's the 2.5 stage bleed on the PW 2037. This bleeds excess air from between the 2nd and 3rd stage of compressor blades for engine stability. It is a very loud sound similar to that of an compressed air gun used to blow off dirt from something. This sound is also very common when the PW powered 757 lands and comes out of reverse thrust .It's a different sound than a afterburner or jet exhaust because the air coming from is relatively cold. Air from jet exhaust or an afterburner is very hot and as it mixes with the cold air around it it makes a distinctly different sound than it would if it was cold. This subject has been discussed at length in the Tech Ops forum if you want more info.
DL757Md

From your description this doesn't sound like a "blown bottle" as the question describes. And can you link to the relevant threads that you refer to?

Thanks for the answer. I've been meaning to return to this question for a while, because frankly I don't do a good job of expressing what I'm looking for. The critical point is that the pitch descends AND THEN RISES. It isn't a Doppler effect; the pitch seems to be dependent on some control surface. But again: I have to state this better. (Thanks again.)

It’s the speed brakes being deployed and then retracted.
The Airbus A320 series makes the exact sound you’re describing. Other medium/large types will make a similar sound.
The reason you hear it in roughly the same place is because approach controllers tend to follow a pattern of speed and altitude changes when sequencing aircraft towards the airport.
Speed brakes are deployed to expedite those speed and altitude changes.

I think this has to be the answer. Here's a clip that has a great example. https://www.youtube.com/watch?v=KoinwyVgk6U

My best guess is that the sound is caused by the pilot lowering the wheels during their descent

This was what I was going to say immediately. I lived about the same distance from the south runway at Pearson and wondered what this was until I saw a 737 drop gear right above me.
I would describe the sound as a sort of "pfff-ewwwww" over the first second, and dropping in pitch until it ends in a constant low rumble after perhaps 5 seconds. Is that it?

Thanks for the answer, but it's been pointed out to me that gear isn't dropped until the planes are quite a bit closer to the ground.

Uhh, I was about the same distance as you, 8 miles right? I could see them dropping their gear above my home.

Then we're in different situations. In my case, the pitch will often go down for a few seconds, hold steady for a few seconds, and then go back up in pitch, shifting over an octave either way.

I think what you hear is the "buzzsaw" caused by tip noise from the fan or the compressor which is only directed out of the engine from certain angles.  Here's a video of the sound from inside

 but because the aircraft is passing you hear it for only a few seconds.

... but then why/how does the pitch change so strongly/rapidly?

The sound exits the engine from the front and only at particular angles. As the aircraft passes overhead the modulates the sound and the velocity of the aircraft modifies the pitch due to the dopper effect. The sound is also modified by reflections from the ground.

It isn't the doppler effect; as the plane passes the relative velocity goes steadily from "rapidly approaching" to "rapidly receding", so the pitch would only go down. In my case, it will go strongly down, then stay stable for a few seconds, and then go up.